Printing presses use plates to print ink onto paper and other media. One method used for creating plates is to expose photosensitive film with the matter to be printed. When the film is developed, the matter imaged on the film may be imaged onto a photosensitive plate, sometimes referred to as "burning" a plate. After processing, the plate can be used on a press to print the matter onto a medium. Part of the plate, usually the part defining the image to be printed, retains ink, while the other part of the plate does not. When the plate is introduced to ink and then to paper or other medium, the image is printed on the medium.
In a black and white printing job, there is usually one plate that is used to print black ink. In a color printing job, a different plate may be used for each color ink. A color job may use three colors of ink, usually cyan, magenta, and yellow, which in combination can be used to make other colors. A plate is usually produced for each color ink. Often, in addition to cyan, magenta, and yellow, black ink is also used. An additional plate is then required to print the black ink. Occasionally, one or more colors will be printed separately as well, referred to as a "spot color." That color will also have its own plate.
Electronic prepress systems have used an imagesetter to receive faster data associated with a plate and to image the raster data onto photosensitive film. In this context, a raster may specify an image by pixels in columns and rows. The film is then used to create a plate. The imagesetter exposes the photosensitive film pixel by pixel. One way that imagesetters image the raster data is to scan a laser across and down a piece of film. Electronics control the laser to expose, or refrain from exposing, each pixel in the raster data. The imagesetter images the pixels on the film in a manner that is precise and repeatable. Recently, platesetters also have been used to create plates directly from raster data without the use of intermediate film. Imagesetters, platesetters and like print engines, including proofers, are also referred to generally as output devices. In this context, imagesetters, platesetters and output devices used to image on material used to make a plate, for example on film and plate, are referred to as final output devices.
Print engines typically have been served by a dedicated raster image processor ("RIP") connected between the print engine and a "front end" computer running imaging application software. An example of such a front end computer is an Apple Macintosh.TM. running Quark Express.TM. imaging software. Other examples include, but are not limited to, Adobe Pagemaker.TM. and Luminous PressWise.TM.. In a typical example configuration, the Macintosh.TM. is connected to an Agfa Atlas.TM. RIP, which communicates with an Agfa Accuset.TM. imagesetter. The RIP interprets the graphic information transmitted to it by the front end, and converts the graphic information into raster data that can be imaged by the print engine. The raster data produced by the RIP typically depends on the capabilities of the imagesetter, such as the imaging resolution and the media type and media size loaded into the imagesetter.
The imaging application software may provide output in the format of a page description language. An example of such a page description language is Postscript.TM. and PDF.TM. offered by Adobe Systems of Mountain View, Calif. Page description languages describe images using descriptions of the objects contained in the page. Use of page description languages allow pages to be described in a way that can be interpreted appropriately for imaging at various sizes and resolutions. Page description language code is generally significantly smaller in data size than the raster data that results from interpreting the page description language code. Use of a page description language therefore allows for transfer of what would be very large image data files over relatively slow serial data communication lines. The same page description language code will be interpreted into different raster data depending on the characteristics of the output device.
When a page description language image file data is received by a RIP, operations performed by the RIP, such as using fonts to lay out text and using color processing to create raster data for each color, results in one or more raster data bit maps. The raster data produced by the RIP is usually binary, meaning that each pixel is either on or off, as distinguished from a contone raster, which includes at least one color level for each pixel. The raster data for each of the colors in a color image are referred to as color "separations." Each separation is transferred from the RIP to the output device over a high speed interface.
When an output device images on film, it is possible to inspect the film before creating a plate. If the image on the film contains an error that may be observed by inspecting the film, such as misaligned, misplaced, or incorrectly included or excluded graphics, text, or colors, the error can be corrected, and a new film imaged. The corrected film may then be used to create the plate. When the output device images directly onto a plate, there is no intermediate film step. It is therefore not possible to observe the results of RIP processing until the plate is made.
When an output device will output directly onto plate, and often when imaging onto film, it is therefore helpful to first image a "preview" version of the image. The quality of the preview image is limited by the quality of preview output devices. Preview output devices are commercially available, and are sometimes referred to as proofers. These output devices generally accept page description language image files as input and produce images that integrate one or more separations onto a single sheet of paper or film. The proofer output, sometimes referred to as a proof image, may resemble the final image output that will be produced by the press. The proof image may be used to verify text, graphics, and color layout. The effectiveness of the proof image is determined by the extent that the proof image resembles the final output.
One reason that a proof image may not resemble the final output is that the page description language interpreter used by a proofer may interpret the same page description language differently than the RIP that provides raster data to the final output device. In this case, there may be significant differences between a final image and the proof image. For example, on the proof image there may be differences in the overlay of image elements resulting in text that may appear to be missing or covered by graphics, while this artifact may not be visible in the final image. As another example, text may appear to be properly placed on the proof image, but be incorrectly placed in the final image.
The problem of having a proof image that is different from the final image reduces the efficiency and adds cost to an electronic prepress system, especially, but not exclusively with the use of direct-to-plate technology.